The invention relates to element frames used in catalyst modules for treating exhaust gases from a stationary combustion source having an exhaust system passing exhaust gases through a support structure containing one or more monoliths, each containing one or more catalysts. A stationary combustion system can be any system that combusts a hydrocarbon-based fuel that is not used in an on-road operated car, truck or aircraft. They can be, for example, coal-fired systems, oil-fired (petroleum) systems or gas turbines. Stationary combustion systems can also be used in marine applications, where combustion systems such as diesel engines, as used for large container or cruise ships. Stationary combustion systems are usually operated continuously under a constant, stationary load while mobile combustion systems are usually operated under varying loads.
Hydrocarbon combustion in these systems, and in engines used in mobile applications, generates exhaust gas that must be treated to remove pollutants like nitrogen oxides (NOx), carbon monoxide (CO) or hydrocarbons (HC) that are formed. NOx is known to cause a number of health issues for humans and animals as well as causing a number of detrimental environmental effects including the formation of smog and acid rain. CO is toxic to humans and animals and HC can cause adverse health effects. To mitigate both the human and environmental impact from these pollutants, especially NOx, in exhaust gas, it is desirable to eliminate these undesirable components, preferably by a process that does not generate other noxious or toxic substances.
Stationary combustion systems can be equipped with an emission control system, which is provided with catalyst modules. FIG. 1 is a depiction of a catalyst module known in the art. Catalyst modules are structures comprising a plurality of element frames, where each element frame can contain a plurality of monoliths each comprising a catalyst support and one or more catalysts. The catalyst modules are installed in a flue gas duct of the emission control system and the flue gas, which is to be purified, flows through the monoliths during operation. The flue gas duct can typically have a cross-sectional area of a few square meters and can be in the tens to hundreds of square meters. The dimensions of the flue duct can vary widely depending upon many factors, including the size of the engine, the conditions under which the engine is operated, permissible back pressure, etc. In some cases, the flue gas duct can have a rectangular cross-section with the width and the height of the duct each being several meters, for example, of 10 m×10 m. The entire cross-sectional area of the flue gas duct is covered by one or more catalyst modules. The catalyst modules are arranged next to one another so that all the flue gas passes through the monoliths, contacts the catalyst(s) on or in the monoliths and becomes purified. Several catalyst modules, for example, two to five, can be placed next to one another in rows and columns, often connected in a supporting framework, within the flue gas duct (FIG. 2). The catalyst modules themselves typically have a rectangular cross-section with an edge length of several meters.
In the direction of flow of the flue gas, catalyst modules frequently are located in several planes positioned one behind the other. In some applications, the catalyst modules can extend for several meters, and even as much as 10-15 meters in the direction of flow (FIG. 3). For some applications, such as marine or gas turbines, relatively harsh ambient conditions in terms of mechanical stress can be present for the catalyst modules. For example, on marine vessels, forces several times gravity can be experienced. In addition, especially for large cross-sections of catalyst modules used with gas turbines, mechanical stresses due to earthquakes have to be considered.
Catalyst modules can be constructed using a stacking frame in which several element frame units are inserted, where the element frame units contain monoliths comprising one or more catalysts. Flue gas flows through the individual monoliths in the direction of the flue gas flow. The monoliths are also known as honeycomb type catalysts. These honeycomb type catalysts are generally made of a ceramic material and have a plurality of flow channels through the monolith in the direction of the gas flow. In the installed, operating state, flue gas flows through the flow channels in the monolith where it interacts with catalyst in the monolith or in a coating on the surface of the monolith and becomes purified.
A typical problem encountered in the use of these treatment systems is that material placed between the monoliths and the element frame elements to provide a seal and therefore, gas-tightness i.e. no bypass flow around the catalyst, as well as, to act as a cushion against vibrations, is not able to stay in place during normal use except for when the material is specially designed and provided as a special type of mat that has a relatively high cost. The mat between element frame and a monolith containing one or more catalysts is placed in the element frame just before the element frame is welded together. In situations where there is periodical mechanical stress, which is typical in exhaust system conditions where engine pulsation leads to shock and vibrations, the mat can move against the frame and monolith. This movement can cause the destruction of the monolith because of the relatively low mechanical stability of the system to shock and vibrations.
Current element frames have metal flaps or lips that overlap the inlet and/or outlet faces of the monolith so that approximately 15% of the catalyst cells are not directly exposed to the exhaust flow.
Current element frames also have locking elements in the center of the inlet and outlet face. One of the functions of the locking element is to shield gaps between the monoliths from direct exhaust gas flow. The locking element is welded without prestress, which leads to relatively low mechanical stability against shock and vibrations.
The current designs of element frames do not provide a mechanism to attach the element frames directly to each other. The ability to join element frames together can avoid the need for a catalyst module when only a small number of element frames are needed.
It would be desirable to have a catalyst module that allows for a cost-effective material to be placed between the monoliths and the metal frame of the element frame to provide a seal and to act as a cushion against vibrations that can stay in place during normal use and also provide for the largest possible catalyst cross-section to be utilized, all under harsh mechanical stress conditions.